In recent years, an organic thin film transistor (organic TFT, i.e., Organic Thin Film Transistor OTFT) draws attention as a next general flat panel display device with high quality and low price or a switching element for driving pixels of an electronic paper.
An organic thin film transistor has substantially the same structure as a silicon thin film transistor, but is different from a silicon thin film transistor in that it employs an organic substance in the semiconductor active layer. The organic thin film transistor can be manufactured without employing a vacuum apparatus according to an ink jet method or a printing method, and therefore, the organic thin film transistor can be manufactured simply and at low cost as compared with a silicon TFT. The organic thin film transistor has advantages that it is suitably applied to an electronic circuit board which is not broken by impact, and can be bent and folded. An organic thin film transistor having such advantages is useful in the case where an element with a wide area is manufactured, a low temperature manufacturing condition is required or a product with high resistance to folding is required, and is desired as an element for driving a matrix of a large size display or an element for driving an organic EL or an electronic paper. Many companies have developed the organic thin film transistor.
Operation principle of an organic thin film transistor is to control resistance by voltage. The gate voltage being controlled, the insulating layer works to generate an accumulation layer in a carrier in a contact interface between the organic semiconductor layer and the insulating layer, whereby current between the two ohm contacts is controlled.
Conventionally, the source electrode, the drain electrode, the gate electrode, the contact electrode or the pixel electrode of an organic thin film transistor has been formed by a vacuum method such as a sputtering method, resulting in cost increase due to the vacuum method.
In order to solve the above problems, an ink jet method (see Patent Document 1 below) or a screen printing method (see Patent Document 2 below) have been investigated as an electrode formation method without employing a vacuum method.
Techniques disclosed in the Patent Documents described above have been studied in detail. As a result, it has been found that in the method disclosed in the Patent Document 1, in which a source electrode and a drain electrode are formed via an ink jet method, controlling affinity of ink to a substrate, additives in the ink remain in the electrodes, which adversely affects transistor performance.
It has been also found that the method disclosed in the Patent Document 2, which forms a source electrode and a drain electrode via a screen printing method employing a silver paste, does not provide source and drain electrode with high resolution nor a transistor with high speed and low power consumption.    Patent Document 1: Japanese Patent O.P.I. Publication No. 2003-318190    Patent Document 2: Japanese Patent O.P.I. Publication No. 2005-72188